When I converted my BJ8 to 3xHD8 I bought three K&N 56-9242 blanks, was just enough room for these. For a standard BJ8 installation I would check if there are room for slightly higher filter.
I made three drawings in scale 1:1 (three offsets used) which I glued inside the end plate using the 2 holes for the studs as reference. Holding it against a light source makes it quite easy to position the drawings. Easy to mark and drill the holes, then use a hole cutter for the intake hole. Used a Q.Max sheet metal punch hole punch diameter 52 mm.
Make sure there are no interference between the screws for the studs inside filter and the carburetor (I changed these screws to insex). Using a gasket for the filter as reference makes it quite simple to check clearance.
For the ventilation breather hose I made a hole in the rear filter end plate, slightly smaller than the hose and just pressed the hose in. Make sure the hose do not interfere with filter element, it is little tight. brg Lars